The idea behind Another Earth first developed out of director Mike Cahill and actress Brit Marling speculating as to what it would be like were one to encounter one’s own self. In order to explore the possibility on a large scale, they devised the concept of a duplicate Earth. The visual representation of the duplicate planet was deliberately made to evoke the Moon, (…) This movie shares some of its plot details with the 1969 British sci-fi movie Doppelganger (imdb).
The film ignores the physical consequences of having a similar-sized planet and moon appear nearby (e.g. effect on tides, gravity and atmosphere) other than depicting night time as brighter due to the reflection of the Sun’s light off the other planet. The DVD / Blu-ray deleted scenes feature reveals that the film makers did intend to illustrate some of the consequences by filming a scene in which Rhoda encounters flowers floating in mid-air, but the scene was cut from the final film. (wiki)